Can two applications listen on the same port at the same time?
We are integrating with an external system which makes to
requests to an agreed URL. In order to learn about the exact format
of the data posted to us, we set up a primitive server written in
Python that logs the request body to a file and responds with a
We kept this server running for several days while we were building
our actual implementation. When we wanted to try out our implementation
we decided to
ssh into the server where the requests where coming
to and port forward the traffic to our development machine.
ssh -R 8090:localhost:8090 user@server)
We expected the port forwarding to fail initially because our primitive server is already listening on the same port. To our great surprise it didn’t fail. The forwarding was working just fine and we could test our implementation. We got disconnected from the server at some point and reconnected again when we needed to do more testing. To an even greater surprise - the primitive server had continued to receive requests after we had disconnected!
This was contrary to anything we had learned so far about sockets
so we decided to investigate deeper how is this possible. We used
netstat to see who is actually listening on that port:
$ sudo netstat -lp ... tcp 0 0 *:8090 *:* LISTEN 32256/python ... tcp6 0 0 localhost:8090 [::]:* LISTEN 4487/1 ...
The primitive server was written in Python and it seems that it binds to all IPv4 interfaces on that machine. However ssh seems to bind to IPv6 localhost interface where no-one is listening on that port.
The system we are integrating with was running on that same machine
and was configured to make requests with URL
It seems that there is a hierarchy in place where
is first picked up by the IPv6 stack and if no-one is listening
there then IPv4 gets a chance.
So when we had the
ssh tunnel in place then IPv6 was used and we
got all the requests to our development machine. However when we
disconnected then IPv4 was used and our primitive logger server
handled the requests.
This is a nice feature of the network stack and something that could be utilized again in the future.